Electrophoretic displays (EPIDS) are now well known. A variety of display types and features are taught in several patents issued in the names of Frank J. DiSanto and Denis A. Krusos and assigned to the assignee herein, Copytele, Inc. of Huntington Station, N.Y. For example, U.S Pat. Nos. 4,655,897 and 4,732,830, each entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS describe the basic operation and construction of an electrophoretic display. U.S. Pat. No. 4,742,345, entitled ELECTROPHORETIC DISPLAY PANELS AND METHODS THEREFOR, describes a display having improved alignment and contrast. U.S. Pat. No. 4,833,464 entitled ELECTROPHORETIC INFORMATION DISPLAY (EPID) APPARATUS EMPLOYING GREY SCALE CAPABILITY relates to an EPID with the capability to display pixels of varying grey scale intensity. This patent recognizes, inter alia, that the duration of application of a voltage gradient at a particular pixel location effects the quantity of pigment particles at that location. Hence, by controlling the time duration of the write pulse one can achieve grey scale capability--the shorter the pulse, the lighter the line.
The display panels shown in the above-mentioned patents operate upon the same basic principle, viz., if a suspension of electrically charged pigment particles in a dielectric fluid is subjected to an applied electrostatic field, the pigment particles will migrate through the fluid in response to the electrostatic field. Given a substantially homogeneous suspension of particles having a pigment color different from that of the dielectric fluid, if the applied electrostatic field is localized, it will cause a visually observable localized pigment particle migration. The localized pigment particle migration results either in a localized area of concentration or rarefaction of particles depending upon the sign and direction of the electrostatic field and the charge on the pigment particles. The electrophoretic display apparatus taught in the foregoing U.S. patents are "triode-type" displays having a plurality of independent, parallel, cathode row conductor elements or "lines" deposited in the horizontal on one surface of a glass viewing screen. A layer of insulating photoresist material deposited over the cathode elements and photoetched down to the cathode elements to yield a plurality of insulator strips positioned at right angles to the cathode elements, forms the substrate for a plurality of independent, parallel column or grid conductor elements or "lines" running in the vertical direction. A glass cap member forms a fluid-tight seal with the viewing window along the cap's peripheral edge for containing the fluid suspension and also acts as a substrate for an anode plate deposited on the interior flat surface of the cap. When the cap is in place, the anode surface is in spaced parallel relation to both the cathode elements and the grid elements. Given a specific particulate suspension, the sign of the electrostatic charge which will attract and repel the pigment particles will be known. The cathode element voltage, the anode voltage, and the grid element voltage can then be ascertained such that when a particular voltage is applied to the cathode and another voltage is applied to the grid, the area proximate their intersection will assume a net charge sufficient to attract or repel pigment particles in suspension in the dielectric fluid. Since numerous cathode and grid lines are employed, there are numerous discrete intersection points which can be controlled by varying the voltage on the cathode and grid elements to cause localized visible regions of pigment concentration and rarefaction. Essentially then, the operating voltages on both cathode and grid must be able to assume at least two states corresponding to a logical one and a logical zero. Logical one for the cathode may either correspond to attraction or repulsion of pigment. Typically, the cathode and grid voltages are selected such that only when both are a logical one at a particular intersection point, will a sufficient electrostatic field be present at the intersection relative to the anode to cause the writing of a visual bit of information on the display through migration of pigment particles. The bit may be erased, e.g., upon a reversal of polarity and a logical zero-zero state occurring at the intersection coordinated with an erase voltage gradient between anode and cathode. In this manner, digitized data can be displayed on the electrophoretic display.
An alternative EPID construction is described in application Ser. No. 07/345,825 entitled DUAL ANODE FLAT PANEL DISPLAY APPARATUS and filed on May 1, 1989 for the assignee herein, which relates to an electrophoretic display in which the cathode/grid matrix as is found in triode-type displays is overlayed by a plurality of independent separately addressable "local" anode lines. The local anode lines are deposited upon and align with the grid lines and are insulated therefrom by interstitial lines of photoresist. The local anode lines are in addition to the "remote" anode, which is the layer deposited upon the anode faceplate or cap as in triode displays. The dual anode structure aforesaid provides enhanced operation by eliminating unwanted variations in display brightness between frames, increasing the speed of the display and decreasing the anode voltage required during Write and Hold cycles, all as explained in application Ser. No. 07/345,825.
A commonly sought objective for EPIDS of both triode and tetrode types, and for digital display equipment and computer and digital apparatus in general, is increased speed of operation. With respect to displays, it is desirable for the display to be able to write, erase and edit the displayed image as quickly as possible in response to operator input and computer processing. For example, when a computer with a visual output device for displaying character information, such as a CRT, is used as a word processor, if the writing and erasure of displayed information is not fast enough, it will slow the operator of the word processor in the completion of his task. Even though the computer memory and processing unit can operate at speeds far exceeding the capacity of a human user, if the input and output devices through which the computer communicates with the user are slow, the computer and the user must wait for the output devices. Thus, if a word processor user is paging through a document at high speed, a slow visual output device may well slow the speed of paging below that at which the user and/or the computer could potentially perform.
In EPIDS and in other display apparatus, because there are a plurality of pixels arranged on a coordinate grid or matrix, and because the pixels must be independently addressable, display operations are frequently conducted at the pixel level, e.g., each pixel is sequentially written to. Sequential operations are intrinsically time consuming, in that the prior operation must be completed before the subsequent can be started. Further, even though the writing of a single pixel can be done very quickly, there are such a large number that even a small write time is significant. A process for independently controlling individual pixel display whereby a degree of parallel display processing is accomplished is described, e.g., in U.S. Pat. No. 4,742,345, wherein display information pertaining to an entire line of pixels, i.e., On or Off, is accumulated in an accumulator or register during a first phase, placed in parallel into a latch array in a second phase and placed in parallel on one of the coordinate grids in a third phase. Placing the display information onto one of the coordinate line sets, e.g., the grid lines which may be oriented in the vertical direction, has been termed "loading" the data on the grid. When the bits of information (voltages corresponding to logical "1" and "0") are placed or "loaded" on, e.g., all the vertical coordinate lines, a single horizontal line can be written by enabling that line, i.e., by placing a voltage corresponding to a logical "1" on that horizontal line. The operation of placing an enabling voltage upon the line to be written, in this case a horizontal cathode line, has been referred to as "writing the line". Of course, this line-by-line writing method also has a upper limit of speed.
With respect to EPIDS, one factor which contributes to the speed with which the display can operate is the speed with which the pigment particles can travel through the electrophoretic fluid under the influence of a particular voltage gradient. Pigment particle migration speed depends, inter alia, upon particle size and electrophoretic fluid viscosity. In addition to the particle speed, there is also the factor of spatial distribution within the EPID envelope, i.e., because the particles are in suspension they are distributed, prior to being exposed to voltage gradients, relatively evenly within the fluid envelope. Accordingly, there is a range of particle proximity to the "target" element, the target element being that element to which the particles are sought to be directed to perform an operation, such as write or erase.
These speed and proximity factors in EPIDS are utilized in U.S. Pat. No. 4,833,464 to control pixel display intensity or grey scale. Namely, if a voltage gradient of shorter or longer duration is applied, fewer or greater particles will accumulate at the "target" electrode thereby affecting pixel intensity, i.e., the greater the number of particles, the greater the intensity. Note that pixel intensity is discernable at both sides of the typical EPID so that an intense accumulation of e.g., light colored particles, on one face of the EPID is accompanied by a correspondingly intense lack of light particles on the other face, which, in all probability, will appear dark due the selection of a dark solution or background for the light colored particles. Thus writing a character on one faceplate of an EPID results in its reverse image being written on the other plate. The writing of a blank character may be termed selective character erasure.
It is an objective of the present invention to provide a method for operating an EPID having any particular pigment particle size, electrophoretic fluid viscosity, electrode arrangement and operating voltage levels, such that the speed of operation is increased.